Fuel-injection systems for modern internal-combustion engines in general comprise a pump designed to send fuel at high pressure to a common rail having a pre-determined volume of accumulation of the fuel, for supplying a plurality of injectors associated to the cylinders of the engine. The pump comprises at least one reciprocating-motion pumping element, which each time carries out a suction stroke and a compression or delivery stroke.
As is known, to obtain a good atomization of the fuel, this must be brought to a very high pressure, for example in the region of 1600 bar in the conditions of maximum load of the engine. Recent standards regarding the limits of the pollutants in the exhaust gases of engines require that the pressure of supply of the fuel to the injectors should be reproducible in the most accurate way possible with respect to what is mapped in the electronic control unit. It is possible to limit the oscillations of the pressure in the common rail with respect to what is envisaged if its volume is more than three orders of magnitude greater than the amount of fuel injected by each injector per combustion cycle. The said common rail is in general very cumbersome, and hence its arrangement on the engine proves critical.
For controlling the pressure in the common rail according to what is mapped in the control unit, it has been proposed to set a by-pass solenoid valve on the delivery pipe of the pump towards the common rail, said valve being controlled by an electronic control unit according to various parameters of operation of the engine. It has also been proposed to perform actuation of the pumping element by means of a cam acting in synchronism with the actuation of each injector.
In these known systems, each pumping element has an instantaneous flow rate, the maximum value of which is much smaller than the maximum flow rate of each injector, so that normally, during an injection event, just a part of the fuel injected, in the region of 20%, is supplied by the pump, whilst the remaining part is supplied by the common rail. Consequently, these systems present the drawback of not requiring the presence of a common rail of adequate dimensions. Furthermore, the pump always works at the maximum flow rate, whilst the by-pass solenoid valve simply discharges into the tank the fuel pumped in excess with respect to what is injected by the injectors, with consequent dissipation of thermal energy.